chapter 2: SDN dataplane fundamentals

Kernel-based Virtual Machine (KVM) is an open source virtualization technology built into Linux. It provides hardware assist to the virtualization software, using built-in CPU virtualization technology to reduce virtualization overheads (cache, I/O, memory) and improving security.

QEMU is a hosted virtual machine emulator that provides a set of different hardware and device models for the guest machine. For the host, QEMU appears as a regular process scheduled by the standard Linux scheduler, with its own process memory. In the process, QEMU allocates a memory region that the guest sees as physical and executes the virtual machine’s CPU instructions.

With KVM, QEMU can just create a virtual machine with virtual CPUs (vCPUs) that the processor is aware of and runs native-speed instructions. When a special instruction is reached by KVM, like the ones that interacts with the devices or to special memory regions, vCPU pauses and informs QEMU of the cause of pause, allowing hypervisor to react to that event.

LibVirt is an Open Source toolkit to manage virtualization platforms. Libvirt is collection of softwares which allow to manage virtual machines and other virtualization functionality, such as storage and network interface management. LibVirt is proposing to define virtual components in a XML-formatted configurations, that are able to be translated into QEMU command line.

Inter process communication (IPC) is a mechanism which allows processes to communicate with each other and synchronize their actions. The communication between these processes can be considered as a method of cooperation between them.

IPC is used in network virtualization in order to be able to exchange data between different distributed processes of a same application (for example, virtio frontend and backend, contrail vrouter agent and dataplane, etc …​) or between processes of distinct applications (e.g., contrail vrouter and QEMU virtio, virtio and VFIO, and so on)

Two different modes of communication are used for IPC:

Like CPU virtualization, two kinds of methods are used to virtualize network devices:

Software Based Emulation are widely supported but can suffer of poor performance. Hardware assisted Emulation if providing good performance thanks to hardware acceleration, but it requires to use a hardware that supports some specific features.

The following two emulated network devices are provided with QEMU/KVM:

Virtio is an open specification for virtual machines' data I/O communication, offering a straightforward, efficient, standard and extensible mechanism for virtual devices, rather than boutique per-environment or per-OS mechanisms. It uses the fact that the guest can share memory with the host for I/O to implement that.

Virtio was developed as a standardized open interface for virtual machines (VMs) to access simplified devices such as block devices and network adaptors.

KVM guests usually have access to software based emulated NIC device (either para-virtualized devices with virtio or traditional emulated devices). On host machines which have Intel VT-d or AMD IOMMU hardware support, another option is possible. PCI devices may be assigned directly to the guest, allowing the device to be used with minimal performance overhead.

Assigned devices are physical devices that are exposed to the virtual machine. This method is also known as passthrough.

The VT-d or AMD IOMMU extensions must be enabled in BIOS in order to be able to perform for device Direct Assignment:

Two methods are supported:

With VFIO the Physical device is exposed to the host user space memory and is made visible from the guest VM it has been assigned.

Single Root I/O Virtualization (SR-IOV) specification is defined by the PCI-SIG (PCI Special Interest Group). This is a PCI Express (PCI-e) that extends a single physical PCI function to share its PCI resources as separate virtual functions (VFs).

The physical function contains the SR-IOV capability structure and manages the SR-IOV functionality (it can be used to configure and control a PCIe device).

A single physical port (root port) presents multiple, separate virtual devices as unique PCI device functions (up to 256 virtual functions – depends on device capabilities).

Each virtual device may have its own unique PCI configuration space, memory-mapped registers, and individual MSI-based interrupts. Unlike a physical function, a virtual function can only configure its own behavior. Each virtual function can be directly connected to a virtual machine via PCI device assignment (passthrough mode).

SR-IOV improves network device performance for each virtual machine as it can share a single physical device between several virtual machines using device direct I/O assignment method.

With SR-IOV, each VM has a direct access to the physical network using the assigned virtual function interface allocated to each. They can communicate altogether using the Virtual Ethernet Bridge provided by the NIC card. A virtual switch can also use SRIOV to get access to the physical network. VM using SRIOV assigned virtual function device has a direct access to the physical network and are not connected to any intermediate virtual network switch or router.

Following command can be used to check whether SR-IOV is supported or not on a physical NIC card:

$ lspci -s <NIC_BDF> -vvv | grep -i "Single Root I/O Virtualization"

VirtIO is bringing lots of flexibility. VirtIO is offering a standardized driver which is fully independent of the hardware used on the physical platform hosting VM instances.

When virtio connectivity is used VM can be easily migrated from one host to another using "live migration" feature. When SRIOV is use, this live migration is not an easy task and is not really possible to achieve.

Indeed, network driver used by VM depends on used hardware on the bare metal node which are hosting them. In order to make VM migration from one bare metal node to another, both nodes must at least to use same hardware NIC model. But when SRIOV is used VM connectivity is having barely the same performance has a real physical NIC, whereas with VirtIO, performance could be poor.

Also, SRIOV, providing a direct access to the physical NIC is making host virtual network nodes (virtual router/switch) used by SDN solution totally blind about VM using such connectivity. Local traffic switching between VM connected on a same SRIOV physical card is achieve by the Virtual Ethernet bridge proposed by SRIOV. Communication between VM connected onto distinct SRIOV physical ports must rely on physical network.

SDN vswitch/vrouter usage is very limited when SRIOV is used. Indeed, packet switching between VMs which are using VFs of a same SR-IOV physical port are using the physical Virtual Ethernet Bridge hosted in the physical NIC.

Only some few use cases are relevant, which are:

With SRIOV we are getting high performance but with poor flexibility and no network virtualization features. With VirtIO we are getting a high level of network virtualization suitable for SDN, which is very flexible with poor performances.

For SDN use cases, we need network virtualization features and performance. DPDK will bring both.

DPDK is dedicating one (or more) CPU to one (or more) thread that are continuously polling a one (or more) DPDK NIC RX queue. CPU on which a DPDK polling thread is started will be loaded at 100% whatever there some packets to process or not, as no interrupt mechanism is used in DPDK to warn the DPDK application that a packet has been received.

Using DPDK library API, physical NIC packets will be made available into user space memory in which the DPDK application is running. So, when DPDK is used there is no user space to kernel space context switching and it saves lots of CPU cycles. Also, the host memory is using large continuous memory area, the huge pages, which allow large data transfers and avoid high data fragmentation in memory which would require a higher memory management effort at the application level. Such a fragmentation would also cost some precious CPU cycles.

Hence, most of the CPU cycles of DPDK pinned CPU are used for polling and processing packets delivered by the physical NIC in DPDK queues. As a result, the packet forwarding task can be processed at a very high speed. If one CPU is not powerful enough to manage incoming packets that are hitting the physical NIC at a very high rate; we can allocate an additional one to the DPDK application in order to increase its packet processing capacity.

A DPDK application is a multi-thread program that is using DPDK library to process network data. In order to scale, we can start several packet polling and processing threads (each one pinned on a dedicated CPU) that are running in parallel.

3 main components are involved into a DPDK application:

Data Plane Development Kit (DPDK) is a set of data plane libraries and network interface controller drivers for fast packet processing, currently managed as an open-source project under the Linux Foundation.

The main goal of the DPDK is to provide a simple, complete framework for fast packet processing in data plane applications.

The framework creates a set of libraries for specific environments through the creation of an Environment Abstraction Layer (EAL), which may be specific to a mode of the Intel® architecture (32-bit or 64-bit), Linux* user space compilers or a specific platform.

These environments are created through the use of make files and configuration files. Once the EAL library is created, the user may link with the library to create their own applications.

The DPDK implements a "run to completion model" for packet processing, where all resources must be allocated prior to calling Data Plane applications, running as execution units on logical processing cores.

The model does not support a scheduler and all devices are accessed by polling. The primary reason for not using interrupts is the performance overhead imposed by interrupt processing.

For more information please refer to dpdk.org documents http://dpdk.org/doc/guides/prog_guide/index.html

DPDK is a set of programing libraries that can be used to create an application that needs to process network packets at a high speed. DPDK is proposing following functions:

In order to reduce Linux user to kernel space context switching all these functions are made available by DPDK into the user space where applications are running. User applications using DPDK libraries have a direct access to the NIC cards, without passing through a NIC Kernel driver as it is required when DPDK is not used.

DPDK is allowing to build user-space multi-thread network application using the POSIX thread (pthread) library.

DPDK is a framework which is made of several libraries:

Ethdev library exposes APIs to use the networking functions of DPDK NIC devices. The bottom half part of ethdev is implemented by NIC PMD drivers. Thus some features may not be implemented.

Poll Mode ethernet Drivers (PMDs) are a key component for DPDK. These PMDs by-pass the kernel and are providing a direct access to the Network Interface Cards (NIC) used with DPDK.

Linux user space device enablers (UIO or VFIO) are provided by Linux Kernel and are required to run DPDK. They are allowing to discover and expose PCI devices information and address space through the /sys directory tree.

DPDK libraries are allowing kernel-bypass application development:

The diagram below is providing an overview of DPDK libraries.

Only few libraries have been described in this diagram: Set of libraries is enriched at each new DPDK release (cf: https://www.dpdk.org/).

The Environment Abstraction Layer (EAL) is responsible to provide access to low-level resources such as hardware and memory space. It provides a generic interface that hides the environment specifics from the applications and libraries. The EAL performs physical memory allocation using mmap() in hugetlbfs (using huge page sizes to increase performance).

Provided services by EAL are:

DPDK has a highly optimized memory manager. DPDK works on a group of fixed size objects called a mempool. Every one of them are pre-allocated. DPDK does not encourage dynamic allocations because it consumes a lot of CPU cycles and it is a speed killer.

DPDK stores incoming packets into mbufs (memory buffers). DPDK pre-allocates a set of mbufs and keeps it in a pool called mempool.

DPDK makes use of mempools each time it needs to allocate a mbuf where packets are stored. Instead of allocating a single mbuf, DPDK do a bulk allocation, or bulk free once packets are consumed. By doing this, packets to be processed (mbufs) are already in cache memory. Therefore, DPDK is very cache friendly.

Mempool has further optimizations. It is very cache friendly. Everything is aligned to the cache and has a some mbufs allocated for each DPDK thread or lcore. Each mempool are also bound with rings which are referencing mbufs containing packets stored into mempool.

Each ring is a highly optimized lockless ring. It can be used by several lcores in a multi-producer/multi-consumer kind of scenario without locks. By avoiding locks, DPDK gets large performance gains, as data structures locking is also a speed killer.

Network Data are stored in compute central memory (in huge page area).

DPDK uses message buffers known as mbufs to store packet data into the host memory. These mbufs are stored in memory pools known as mempools.

mbufs are storing DPDK NIC incoming and outgoing packets which have to be processed by the DPDK application.

DPDK queues are not storing the packets but a pointer onto the real packet. It avoids performing a data transfer that would be needed when packets have to be forward from a DPDK NIC to another.

Packets are not moved from one queue to another, but these are descriptors (pointers) that are moving from one queue to another.

Descriptors are set up as a ring. A ring is a circular array of descriptors. Each ring describes a single direction DPDK NIC queue. Each DPDK NIC queue is made up of 2 rings (1 per direction: 1 RX ring, 1 TX ring).

Each descriptor points onto a packet that has been received (RX ring) or that is going to be transmitted (TX ring).

The more descriptors RX/TX rings are containing, the more memory size will be required in each mempool (number of mbufs) to store data.

DPDK application is only processing packets that are exposed in user space host OS memory.
DPDK rings are an abstraction of the real NIC queues: DPDK is using DMA to keep synchronized at anytime between the NIC hardware queues and its DPDK representation in the host memory.

Multithreading is the ability of a CPU (single core in a multi-core processor architecture) to provide multiple threads of execution concurrent. In a multithreaded application, the threads share some CPU resources memory:

A single Linux process can contain multiple threads, all of which are executing the same program. These threads share the same global memory (data and heap segments), but each thread has its own stack (local variables).

Linux pThreads (POSIX threads) is a C library which contains a set functions that are allowing to manage threads into an application. DPDK is using Linux pThreads library.

DPDK is using threads that are designed as "lcore”. A “lcore" refers to an EAL thread, which is really a Linux pthread, which is running onto a single processor execution unit.

Lcores are not sharing CPU units. Nevertheless, if the host processor supports hyperthreading, a core may include several lcores or threads.

lcores are used to run DPDK application packet processing threads. Several packet processing models are proposed by DPDK. The simplest one is the Run-To-Completion model.

Run-to-Completion, is using a single thread (lcore) for end to end packet processing (packet polling, processing and forwarding).

A complex application is typically split across multiple cores, with cores communicating through Software queues.

Packet Framework facilitates the creation of pipelines. Each pipeling thread is assigned to a CPU and is using software queues like output or/and input ports.

For instance, Contrail DPDK vRouter is using such a model for GRE encapsulated packet processing.

It is possible to create Control Threads. Those threads can be used for management/infrastructure tasks and are used internally by DPDK for multi process support and interrupt handling.

DPDK service cores enables a dynamic way of performing work on DPDK lcores. Service core support is built into the EAL, and an API is provided to optionally allow applications to control how the service cores are used at runtime.

PCI devices have a set of registers referred to as configuration space for devices. These configuration space registers are mapped to host memory locations.

When a PCI device is enabled, the system’s device drivers (by writing configuration commands to the PCI controller) programs the Base Address Registers (BAR) to inform the PCI device of its address mapping. Next, the host operating system is able to address this PCI device.

With usual Linux NIC Kernel, both NIC configuration and Packet processing is done in Kernel Space. User applications which have to establish a TCP connection or send a UDP packet is using the sockets API, exposed by libc library.

Linux Packet Processing with sockets API is requiring following operations which are costly:

With usual Linux Drivers most of operations are occurring in Kernel modes and are requiring lots of user space to kernel space context switching and interruption mechanisms. The heavy context switching usage is costing lots of CPU cycles and is a limiting the numbers of packets that a CPU is able to process. Such drivers are not able to perform packet processing at expected high speed, especially when 10/40/100G Ethernet generation cards are used on a Linux System.

A Poll Mode Driver consists of APIs, running in user space, to configure the devices and their respective queues. In addition, a PMD accesses the RX and TX descriptors directly without any interrupts (with the exception of Link Status Change interrupts) to quickly receive, process and deliver packets in the user’s application.

Poll Mode drivers are involved in NIC configuration. They are exposing NIC configuration registers into host memory area which is directly reachable from user space.

In short, Poll Mode Drivers are user space pthreads which:

Hence user applications can configure directly the NIC cards they are using from Linux user space where they are running.

A first configuration phase is using Poll Mode Drivers and DPDK library to configure DPDK rings buffers into Linux user space. Next, incoming packets will be automatically transferred with DMA (Direct Memory Access) mechanism from NIC physical RX queues in NIC memory to DPDK RX rings buffer in host memory. DMA (Direct Memory Access) is also used to transfer outgoing packets from DPDK TX rings buffer in host memory to NIC physical TX queues in NIC memory. DMA offloads expensive memory operations, such as large copies or scatter-gather operations, from the CPU.

Direct Memory Access (DMA) allows PCI devices to read (write) data from (to) memory without CPU intervention. This is a fundamental requirement for high performance devices.

DMA is a mechanism that is using a specific hardware controller to manage read and write operations into the main system memory (RAM: Random Access Memory). This mechanism is totally independent of the central processing unit (CPU) and does not consume any CPU resource. A DMA transfer is used to manage data transfer. DMA transfer is triggered by the CPU and is working in background using the specific hardware resource (DMA controller).

DPDK rings and NIC buffers are synchronized with DMA. Thanks to this synchronization mechanism, DPDK application can access transparently to NIC packets in user space reading or writing data in DPDK rings.

Input–Output Memory Management Unit (IOMMU) is a memory management unit (MMU) that connects a Direct Memory Access (DMA) capable I/O bus to the main memory.

In Virtualization, an IOMMU is re-mapping the addresses accessed by the hardware into a similar translation table that is used to map guest virtual machine address memory to host-physical addresses memory.

IOMMU provides a short path for device to get access only to a well scoped physical device memory area which corresponds to a given guest virtual machine memory. IOMMU helps to prevent DMA attacks that could be originated by malicious devices. IOMMU provides DMA and interrupt remapping facilities to ensure I/O devices behave within the boundaries they’ve been allotted.

Intel has published a specification for IOMMU technology as Virtualization Technology for Directed I/O, abbreviated as VT-d.

In order to get IOMMU enabled:

GRUB configuration example with IOMMU Passthrough enabled:

DPDK Library includes Poll Mode Drivers (PMDs) for physical and emulated Ethernet controllers which are designed to work without asynchronous, interrupt-based signaling mechanisms.

Lots of other NIC are supported by DPDK (cf http://doc.dpdk.org/guides/nics/overview.html).

Different PMDs may require different kernel drivers in order to work properly (cf Linux User space device enablers). Depending on the PMD being used, a corresponding kernel driver should be loaded and bound to the network ports.

This is also preferable that each NIC has been flashed with the latest version of NVM/firmware.

Linux kernel version 2.6 introduced the User Space IO (UIO) loadable module. UIO is a kernel-bypass mechanism which provides an API that enables user space handling of legacy interrupts (INTx).

UIO has some limitations:

UIO only supports legacy interrupts so it is not usable with SR-IOV and virtual hosts which require MSI/MSI-X interrupts.

Despite these limitations, UIO is well suited for use in virtual machines, where direct IOMMU access is not available. In such a situation, a guest instance user space process is not isolated from other processes in the same instance. But the hypervisor can isolate any guest instance from others or hypervisor host processes using IOMMU.

Currently, two UIO modules are supported by DPDK:

DPDK specific UIO Kernel module is loaded with insmod command after UIO module has been loaded:

While a single command is needed to load Linux Generic UIO module:

DPDK specific UIO module could be preferred in some situation to Linux Generic UIO module (cf: https://doc.dpdk.org/guides/linux_gsg/linux_drivers.html)

Virtual Function I/O (VFIO) kernel infrastructure was introduced in Linux version 3.6.

VFIO provides a user space driver development framework allowing user space applications to interact directly with hardware devices by mapping the I/O space directly to the application’s memory.

VFIO is a framework for building user space drivers that provides:

VFIO exposes APIs which allow to:

VFIO driver is an IOMMU/device agnostic framework for exposing direct device access to user space, in a secure, IOMMU protected environment. For bare-metal environments, VFIO is the preferred framework for Linux kernel-bypass. It operates with the Linux kernel’s IO.

MMU subsystem is used to place devices into IOMMU groups. User space processes can open these IOMMU groups and register memory with the IOMMU for DMA access using VFIO ioctl calls. VFIO also provides the ability to allocate and manage message-signaled interrupt vectors.

A single command is needed to load VFIO module:

Despite VFIO has been created to work with IOMMU, VFIO can be also be used without (this is just as unsafe as using UIO).

VFIO is generally the preferred Linux user space device enabler to be used because it supports IOMMU to protect host memory. When a real hardware PCI device is attached to host system and IOMMU is used with VFIO, all the reads/writes of that device done in user space by the DPDK application will be protected by the host IOMMU.

But there some is few exceptions. Below is Intel recommendation for the choice of the Kernel driver to be used with DPDK:

https://software.intel.com/content/www/us/en/develop/articles/memory-in-dpdk-part-2-deep-dive-into-iova.html

NUMA means Non-Uniform Memory Access systems

A traditional server has a single CPU, a single RAM and a single RAM controller.

A RAM can be made of several DIMM banks in several sockets, all being associated to the CPU. When the CPU needs access to data in RAM, it requests it to its RAM controller.

Recent servers can have multiple CPUs, each one having its own RAM and its own RAM controller. Such systems are called NUMA systems, or Non-Uniform Memory Access. For example, in a server with 2 CPUs, each one can be a separate NUMA: NUMA0 and NUMA1.

NUMA nodes architecture.

When CPU0 needs to access data located in RAM0, it will go through its local RAM controller 0. Same thing happens for CPU1.

When CPU0 needs to access data located in the other RAM1, the first (local) controller 0 has to go through the second (or remote) RAM controller 1 which will access the (remote) data in RAM 1. Data will use an internal connection between the 2 CPUs called QPI, or Quick Path Interconnect, which is typically of a high enough capacity to avoid being a bottleneck, typically 1 or 2 times 25GBps (400 Gbps). For example, the Intel Xeon E5 has 2 CPUs with 2 QPI links between them; Intel Xeon E7 has 4 CPUs, with a single QPI between pairs of CPUs.

The fastest RAM that the CPU has access to is the register, which is inside the CPU and reserved to it.

Beyond the register, the CPU has access to cached memory, which is a special memory based on higher performance hardware.

Cached memories are shared between the cores of a single CPU. Typical characteristics of memory cache are:

If the CPU needs to access data that is in the main RAM, it has to use its RAM controller.

Access to RAM takes typically 170 CPU cycles (the green line in the diagram). Access to the remote RAM through the remote RAM controller typically adds 200 cycles (the red line in the diagram), meaning RAM latency is roughly doubled.

When data needed by the CPU is located both in the local and in the remote RAM with no particular structure, latency to access data can be unpredictable and unstable.

A single physical CPU core with hyper-threading appears as two logical CPUs to an operating system.

While the operating system sees two CPUs for each core, the actual CPU hardware only has a single set of execution resources for each core.

Hyper-threading allows the two logical CPU cores to share physical execution resources.

The sharing of resources allows two logical processors to work with each other more efficiently and allows a logical processor to borrow resources from a stalled logical core (assuming both logical cores are associated with the same physical core). Hyper-threading can help speed processing up, but it’s nowhere near as good as having actual additional cores.

Memory is managed in blocks known as pages. On most systems, a page is 4KB. 1MB of memory is equal to 256 pages; 1GB of memory is 256,000 pages, etc. CPUs have a built-in memory management unit that manages a list of these pages in hardware.

The Translation Lookaside Buffer (TLB) is a small hardware cache of virtual-to-physical page mappings. If the virtual address passed in a hardware instruction can be found in the TLB, the mapping can be determined quickly. If not, a TLB miss occurs, and the system falls back to slower, software-based address translation. This results in performance issues. Since the size of the TLB is fixed, the only way to reduce the chance of a TLB miss is to increase the page size.

Virtual memory address lookup slows down when the number of entries increases.

A huge page is a memory page that is larger than 4Ki. In x86_64 architecture, in addition to standard 4KB memory page size, two larger page sizes are available: 2MB and 1GB.

Contrail DPDK vrouter can use both or only one huge page size.

An Operating System is using a scheduler to place each single process and/or threads it has to run onto one CPUs offered by a host.

There are two kinds of scheduling, cooperative and preemptive. By default, Linux scheduler is using a cooperative mode.

In order to get a CPU booked for a subset of tasks, we have to inform the Operating System scheduler not to use these CPUs for all the tasks it has to run.

These CPUs are told: "isolated" because they are no more used by the OS to process all tasks. In order to get a CPU isolated several mechanisms can be used:

Isolation and pinning are two complementary mechanisms that are proposed by Linux OS:

Tasks to be run by an operating system must be spread across available CPUs. These tasks in a multi-threading environment are often made of several processes which are also made of several threads.

A thread’s CPU affinity mask determines the set of CPUs on which it is eligible to run.

Linux systemd is a software suite that provides an array of system components for Linux operating systems. Its primary component is an init system used to bootstrap user space and manage user processes.

CPUAffinity parameter restricts all processes spawned by systemd to the list of cores defined by the affinity mask.

taskset is used to set or retrieve the CPU affinity of a running process given its PID or to launch a new COMMAND with a given CPU affinity.

We can retrieve the CPU affinity of an existing task:

Or set it:

Virtual IOMMU (vIOMMU) is allowing to emulate IOMMU for guest VMs.

vIOMMU has the following characteristics:

In order to get a virtual device working with a virtual IOMMU we have to:

Following mechanisms can be used to create vIOMMU memory mappings:

The integration between the virtual IOMMU and any user space network application like DPDK is usually done through the VFIO driver. This driver will perform device isolation and automatically add the memory (IOVA -to GPA) mappings to the virtual IOMMU.

The use of hugepages memory in DPDK contributes to optimize TLB lookups, since a fewer number of memory pages can cover the same amount of memory. Consequently, the number of Device TLB synchronization messages drop dramatically. Hence, the performance penalty TLB lookups is lowered.

Cf: https://www.redhat.com/en/blog/journey-vhost-users-realm

https://wiki.qemu.org/Features/VT-d

Virtio-pmd driver, is a DPDK driver, built on the Poll Mode Driver abstraction, that implements the virtio protocol.

Vhost user protocol moves the virtio ring from kernel all the way to userspace. The ring is shared between the guest and DPDK application. QEMU sets up this ring as a control plane using unix sockets.

If the both the host server guest virtual machine are DPDK there are no VMExits in the host for guest packets processing. Guest virtual machine uses virtio-net PMD driver and performs packets polling. So. There is nothing running in kernel here, so there are no system calls. Since both system calls and VM Exits are avoided, the performance boosts significantly. It will be an order higher.

When a DPDK application is running into a guest Virtual Machine, a mechanism has to be used to expose one of the host physical NIC to this guest in order it gets access to the physical network.

IOMMU protects host memory against malicious or bug writes which can corrupt host memory at any time. But, when a physical device is assigned to a guest virtual machine without vIOMMU usage, the guest memory address space is totally exposed to the hardware PCI device.

A PCI device can be assigned to a guest in order to be used by a guest DPDK application. By leveraging VFIO driver in the host kernel we provide a direct access to an assigned physical NIC from this guest protected with IOMMU.

Next, by leveraging VFIO driver in the guest kernel we provide a direct access to the assigned physical from this guest user space. vIOMMU is providing a secure mechanism to manage DMA transfer between an assigned physical hardware and hosted guest virtual instance memory area.

This use case is almost the same as PCI passthrough. VFIO and IOMMU are used to expose a SRIOV virtual function directly to a guest VM.

An additional Physical function driver which is vendor specific is used to manage the virtual function creation on the physical NIC. This driver is used by a Virtual Machine Manager (like libvirt) to create the virtual function before the virtual instance is spawned.

Physical incoming packets are directly copied in guest memory without involving the host server. SR-IOV only allow to share a physical NIC between several guests but does not change the packet processing path provided by PCI passthrough.

With virtio full hardware offloading, both the virtio data plane and virtio control plane are offloaded to the NIC hardware. The physical NIC must support:

Hence once the guest memory is mapped with the NIC using virtio physical device passthrough, the guest communicates directly with the NIC via PCI without involving any specific drivers in the host kernel.

Guest VM packet processing is directly performed in NIC hardware like but presented to the guest instance like a regular virtio emulated interface. Guest VM does not make any difference between a virtio emulated interface and an assigned physical virtio NIC, as they are exposed with the same virtio driver frontend in the guest.

virtio device passthrough

Virtio device passthrough can be implemented onto a NIC which is supporting or not SR-IOV.

Like other physical device assignment technics presented in this book, VFIO and IOMMU are used to present the physical device NIC into the guest VM user space.

Hence, such a virtio physical NIC can be used by a DPDK application running into a virtual instance. But, like other virtio device passthrough has also the same limitations for SDN. As the host operating system is totally by passed by this mechanism, we cannot interconnect instances using such NIC interface with a SDN virtual router or switch.

The main advantage of Virtio device passthrough is the flexibility it provides for a virtual instance to use transparently either a real physical interface or an emulated one. It offers an Open public specification, which provide device fully independent of any specific vendor.

Virtio full HW offloading, can support live migration thanks to virtio, which is not possible to achieve without any specific implementation with SR-IOV.

But in order to be able to support such a feature, latest virtio specifications (1.1 version) must be implemented onto both QEMU and the NIC hardware used on the cloud infrastructure.

Like full hardware offloading, virtual Data Dath Acceleration (vDPA) aims to:

vDPA is presenting a generic control plane through a software piece which provides an abstraction layer on top of physical NIC.

Like Virtio full hardware offloading, vDPA build a direct data path between the gest network interface and the physical NIC, using the virtio ring layout. But for the control path a generic vDPA driver (mediation driver) is used to translate the vendor NIC driver/control-plane to the VirtIO control plane, in order to allow each NIC vendor to keep using its own driver.

It allows NIC vendors to support virtio ring layout at smaller effort keeping wire speed performance on the data plane.

virtio datapath acceleration

vDPA is requiring a vendor specific "mediation device driver" to be loaded in the host operating system.

A NIC card generation commonly named "smart NIC" are highly customizable thanks to the last evolution provided by some new capabilities (FPGA, P4).

It makes possible to envisage SDN vSwitch/vRouter dataplane function to be moved into the NIC card keeping only the controle plane function into the host operating system.

For Contrail solution, this is made by offloading several Contrail vRouter tables including:

It allows to accelerate lookups and forwarding actions that are directly performed into the NIC.

SDN packet processing is fully done into the NIC card, no more host CPU processing is involved in packet processing.

Two implementations are proposed by Metronome:

SRIOV + SmartNIC:

vDPA + Smart NIC:

Berkeley Packet Filter (BPF) was designed for capturing and filtering network packets that matched specific rules. In last years extended BPF (eBPF) has been designed to take advantage of new hardware (64 bits usage for intance). An eBPF program is "attached" to a designated code path in the kernel.

eXpress Data Path (XDP), uses eBPF to achieve high-performance packet processing by running eBPF programs at the lowest level of the network stack, immediately after a packet is received. XDP.

XDP support is made available in the Linux Kernel since version 4.8, while eBPF is supported in the Linux Kernel since version 3.18.

XDP requires:

XDP packet processor performs:

BFP programs:

BFP program main actions are :

XDP is also able to offload an eBPF program to a NIC card which supports it, reducing the CPU load.

XDP and eBPF does not require:

PS: eBPF rules are also supported in DPDK application.

https://www.redhat.com/en/blog/using-express-data-path-xdp-red-hat-enterprise-linux-8